In the prior art it is well known to solder, fuse or braze articles by condensing hot saturated vapor of a heat transfer liquid thereon. Typically such operations are performed in a chamber having a reservoir of the heat transfer liquid therein. Immersion heating coils in the reservoir are activated to boil the liquid while cooling coils mounted in the upper portion of the chamber are used to condense vapor from the boiling liquid. Such an arrangement forms a confined body of hot saturated vapor by substantially precluding the escape of the vapor from the chamber. The articles are then immersed in the body of vapor which condenses thereon and gives up its latent heat of vaporization thereto to heat the articles to the temperature required for soldering, brazing or fusing. Such a technique is described in U.S. Pat. No. Re. 30,399 which issued on Sept. 9, 1980 and is assigned to Western Electric Company and Bell Telephone Laboratories, Inc. and is incorporated by reference herein.
This technique has proven to be eminently successful, however, solder flux placed on the article to be soldered is washed therefrom by the liquid condensate and falls into the reservoir of the boiling heat transfer liquid. The flux and other contaminants coming in contact with the immersion heaters can cause "hot spots" on the heaters which can lead to thermal degradation of the heat transfer fluid. Also, the presence of large quantities of flux in the heat transfer fluid accelerates the formation of acid within the vapor environment which leaves undesirable ionic contamination on processed parts and shortens the useful life of the chamber.
U.S. Pat. No. 4,077,467 describes a system for removing the flux from the heat transfer liquid. The mixture of the high temperature (e.g., 419.degree. F.) liquid with the flux suspended therein is drained from a reservoir into a cooling chamber, the flux thereby becoming solidified in suspension. The suspended, solidified flux is then filtered out by a mechanical filter at the output line of the cooling chamber. The heat transfer liquid passing through the filter is then pumped back into the reservoir.
A further addition that has been used with such a filtering arrangement is a pan located within the body of vapor located between the heat transfer liquid and the position of the article as it is being soldered. The flux and the heat transfer liquid drain by gravity from the article into the pan which has a conduit that directs the liquid into the cooling chamber, through the filters and back into the reservoir.
Such techniques result in a periodic passing of the flux contaminated heat transfer liquid through the system. That is, the condensing liquid and flux will drain from the soldered article, during the solder reflow cycle, and pass into the filtration system as the average temperature of the article approaches the vapor temperature of the heat transfer fluid. The flow of condensate through the filtration system is rapid at the beginning of each solder reflow cycle when the temperature difference between process part and vapor is maximum. As the article temperature approaches the temperature of the vapor, the condensate flow to the filtration system is reduced. Ultimately, the article reaches thermal equilibrium with the vapor and the condensate flow subsides entirely, the soldered article is then removed from the facility and the cycle repeated. Accordingly, the flux contaminated heat transfer liquid pulses through the filtration system periodically.
The pulsed flow of flux contaminated liquid through the system has been found to clog conduits, valves, pumps and heat exchangers in existing filtration systems. Apparently the clogging is due to flux adhering to the relatively cooler surfaces throughout the filtration system prior to removal of the contaminants by mechanical filter cartridges. Such clogging results in an inefficient system wherein liquid flow therethrough becomes restricted and necessitates increased maintenance and cleaning of both the filters and condensation facility.
Accordingly, there exists a need for an efficient flux filtration system that requires minimal maintenance and cleaning.